Memory system

ABSTRACT

A memory system includes a DRAM  20  that performs writing and readout in a unit equal to or smaller than a cluster, a NAND memory  10  that performs writing and readout in a page unit, and a management table group in which management information including storage locations of data stored in the DRAM  20  and the NAND memory  10  is stored. When a readout request is received from the outside, a data managing unit  120  notifies, when an unwritten logical address area is present in a storage area of the NAND memory to which a logical address area requested to be read out is mapped, fixed data stored in the DRAM  20  to the outside in association with the logical address area.

TECHNICAL FIELD

The present invention relates to a memory system including a nonvolatilesemiconductor memory.

BACKGROUND ART

In recent years, nonvolatile semiconductor storage devices such as aflash memory for storing information according to an amount ofaccumulated charges are widely known. Recently, a capacity of a NANDflash memory is increased. A personal computer incorporating the NANDflash memory as a secondary storage device is put to practical use.

The NAND flash memory is a semiconductor memory in which eraseprocessing is necessary before writing data. The durable life of theNAND flash memory depends upon the number of times of erasing (thenumber of times of rewriting). Data erasing and data writing for theNAND flash memory are performed by applying high voltage between asubstrate and a control gate to inject electrons into and dischargeelectrons from a floating gate. Therefore, when erase processing andwrite processing are performed a large number of times, a gate oxidefilm around the floating gate is deteriorated and the electrons injectedinto the floating gate are lost. It is likely that recorded data isbroken.

Data recorded by a computer such as a personal computer has temporallocality and regional locality (see, for example, Non-Patent Document1). Because of this characteristic, if data is directly recordedaccording to an address designated from the outside, rewrite processingand erase processing are concentrated in a specific area and respectivestorage devices are unequally deteriorated. Therefore, in the NAND flashmemory of this type, processing called wear leveling for equallydistributing data update sections in a semiconductor storage device isperformed to generally equalize the number of times of erasing in allmemory cells in the semiconductor storage device.

Further, in the NAND flash memory, a phenomenon called read disturb inwhich data in a read-out page and data in another page in the same blockas the read-out page are unstable. The read disturb is a phenomenon inwhich, during data readout, because of word line potential (unselectedword line potential) applied to a control gate of an unselected cell,for example, electrons are injected into a floating gate of a cellhaving a value “1” and the value of the cell is changed to “0”.

On the other hand, it is said that unlikelihood of the read disturb,i.e., durability against the read disturb is deteriorated as erasing andwriting of data are repeated a larger number of times (see, for example,Patent Document 1). It is also said that the read disturb substantiallydepends upon the number of times of readout from a memory cell. Further,it is also said that, in a multi-value storage device in which 2-bit ormore information can be stored in one memory cell, the influence of theread disturb increases because intervals of thresholds for identifyingvalues of stored data are narrow. Therefore, the phenomenon called readdisturb is a serious problem for a storage device.

[Patent Document 1] Japanese Patent Document Laid-Open No H10-228783

[Non-Patent Document 1] David A. Patterson and John L. Hennessy,“Computer Organization and Design: The Hardware/Software Interface”,Morgan Kaufman Pub, Aug. 31, 2004

DISCLOSURE OF INVENTION

One aspect of this invention is to provide a memory system comprising:

a first storing area as a cache memory including a volatilesemiconductor memory; second and third storing areas includingnonvolatile semiconductor memories in which readout and writing areperformed in a page unit and erasing is performed in a block unit twiceor a larger natural number times as large as the page unit; a managementtable group in which management information including storage locationsof data stored in the first, second, and third storing areas is stored;and a controller that allocates a storage area of the nonvolatilesemiconductor memory to the second and third storing areas in a logicalblock unit associated with one or more of the blocks, wherein when areadout request is received from outside, the controller determines,based on the management table group, whether an unwritten logicaladdress area is present in the second or third storage area to which alogical address area requested to be read out is mapped and notifies,when the unwritten logical address area is present, predetermined fixeddata to the outside in association with the unwritten logical addressarea.

One aspect of this invention is to provide a memory system comprising:

a first storing area as a cache memory including a volatilesemiconductor memory; second and third storing areas includingnonvolatile semiconductor memories in which readout and writing areperformed in a page unit and erasing is performed in a block unit twiceor a larger natural number times as large as the page unit; a managementtable group in which management information including storage locationsof data stored in the first, second, and third storing areas is stored;and a controller that allocates a storage area of the nonvolatilesemiconductor memory to the second and third storing areas in a logicalblock unit associated with one or more of the blocks, wherein when awrite request involving data flushing from the first storing area to thesecond storing area is received from outside and all data of a logicaladdress area including the page unit flushed from the first storing areato the second storing area are not present in the first storing area,the controller determines, based on the management table, whether anunwritten logical address area is present in the second and thirdstoring areas to which a logical address area used for padding data notpresent in the first storing area is mapped and selects, when theunwritten logical address area is present, predetermined fixed data aspadding source data.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a configuration example of an SSD;

FIG. 2 is a diagram of a configuration example of one block included ina NAND memory chip and a threshold distribution in a quaternary datastorage system;

FIG. 3 is a block diagram of a hardware internal configuration exampleof a drive control circuit;

FIG. 4 is a block diagram of a functional configuration example of aprocessor;

FIG. 5 is a block diagram of a functional configuration formed in a NANDmemory and a DRAM;

FIG. 6 is a detailed functional block diagram related to writeprocessing from a WC to the NAND memory;

FIG. 7 is a diagram of an LBA logical address;

FIG. 8 is a diagram of a configuration example of a management table ina data managing unit;

FIG. 9 is a diagram of an example of an RC cluster management table;

FIG. 10 is a diagram of an example of a WC cluster management table;

FIG. 11 is a diagram of an example of a WC track management table;

FIG. 12 is a diagram of an example of a track management table;

FIG. 13 is a diagram of an example of an FS/IS management table;

FIG. 14 is a diagram of an example of an MS logical block managementtable;

FIG. 15 is a diagram of an example of an FS/IS logical block managementtable;

FIG. 16 is a diagram of an example of an intra-FS/IS cluster managementtable;

FIG. 17 is a diagram of an example of a logical-to-physical translationtable;

FIG. 18 is a flowchart of an operation example of read processing;

FIG. 19 is a flowchart of an operation example of write processing;

FIG. 20 is a diagram of combinations of inputs and outputs in a flow ofdata among components and causes of the flow;

FIG. 21 is a flowchart of a processing flow of processing for reducingNAND memory accesses during read processing; and

FIG. 22 is a flowchart of a processing flow of processing for reducingNAND memory accesses during write processing.

FIG. 23 is a perspective view of an example of a personal computer; and

FIG. 24 is a diagram of an example of system architecture in a personalcomputer.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Exemplary embodiments of Memory System according to the presentinvention will be explained below in detail with reference to theaccompanying drawings. The present invention is not limited to thefollowing embodiments.

Embodiments

Embodiments of the present invention are explained below with referenceto the drawings. In the following explanation, components having thesame functions and configurations are denoted by the same referencenumerals and signs. Redundant explanation of the components is performedonly when necessary.

First, terms used in this specification are defined.

Physical page: A unit that can be collectively written and read out in aNAND memory chip. A physical page size is, for example, 4 kB. However, aredundant bit such as an error correction code added to main data (userdata, etc.) in an SSD is not included. Usually, 4 kB+redundant bit(e.g., several 10 B) is a unit simultaneously written in a memory cell.However, for convenience of explanation, the physical page is defined asexplained above.

Logical page: A writing and readout unit set in the SSD. The logicalpage is associated with one or more physical pages. A logical page sizeis, for example, 4 kB in an 8-bit normal mode and is 32 kB in a 32-bitdouble speed mode. However, a redundant bit is not included.

Physical block: A minimum unit that can be independently erased in theNAND memory chip. The physical block includes a plurality of physicalpages. A physical block size is, for example, 512 kB. However, aredundant bit such as an error correction code added to main data in theSSD is not included. Usually, 512 kB+redundant bit (e.g., several 10 kB)is a unit simultaneously erased. However, for convenience ofexplanation, the physical block is defined as explained above.

Logical block: An erasing unit set in the SSD. The logical block isassociated with one or more physical blocks. A logical block size is,for example, 512 kB in an 8-bit normal mode and is 4 MB in a 32-bitdouble speed mode. However, a redundant bit is not included.

Sector: A minimum access unit from a host. A sector size is, forexample, 512 B.

Cluster: A management unit for managing “small data (fine grained data)”in the SSD. A cluster size is equal to or larger than the sector size,and for example, is set such that a size twice or larger natural numbertimes as large as the cluster size is the logical page size.

Track: A management unit for managing “large data (coarse grained data)”in the SSD. A track size is set such that a size twice or larger naturalnumber times as large as the cluster size is the track size, and forexample, a size twice or larger natural number times as large as thetrack size is the logical block size.

Free block (FB): A logical block on a NAND-type flash memory for which ause is not allocated. When a use is allocated to the free block, thefree block is used after being erased.

Bad block (BB): A physical block on the NAND-type flash memory thatcannot be used as a storage area because of a large number of errors.For example, a physical block for which an erasing operation is notnormally finished is registered as the bad block BB.

Writing efficiency: A statistical value of an erasing amount of thelogical block with respect to a data amount written from the host in apredetermined period. As the writing efficiency is smaller, a weardegree of the NAND-type flash memory is smaller.

Valid cluster: A cluster that stores latest data corresponding to alogical address.

Invalid cluster: A cluster that stores non-latest data not to bereferred as a result that a cluster having identical logical address iswritten in other storage area.

Valid track: A track that stores latest data corresponding to a logicaladdress.

Invalid track: A track that stores non-latest data not to be referred asa result that a cluster having identical logical address is written inother storage area.

Compaction: Extracting only the valid cluster and the valid track from alogical block in the management object and rewriting the valid clusterand the valid track in a new logical block.

First Embodiment

FIG. 1 is a block diagram of a configuration example of an SSD (SolidState Drive) 100. The SSD 100 is connected to a host apparatus 1 such asa personal computer or a CPU core via a memory connection interface suchas an ATA interface (ATA I/F) 2 and functions as an external storage ofthe host apparatus 1. The SSD 100 can transmit data to and receive datafrom an apparatus for debugging and manufacture inspection 200 via acommunication interface 3 such as an RS232C interface (RS232C I/F). TheSSD 100 includes a NAND-type flash memory (hereinafter abbreviated asNAND memory) 10 as a nonvolatile semiconductor memory, a drive controlcircuit 4 as a controller, a DRAM 20 as a volatile semiconductor memory,power supply circuit 5, an LED for state display 6, a temperature sensor7 that detects the temperature in a drive, and a fuse 8.

The power supply circuit 5 generates a plurality of different internalDC power supply voltages from external DC power supplied from a powersupply circuit on the host apparatus 1 side and supplies these internalDC power supply voltages to respective circuits in the SSD 100. Thepower supply circuit 5 detects a rising edge of an external powersupply, generates a power-on reset signal, and supplies the power-onreset signal to the drive control circuit 4. The fuse 8 is providedbetween the power supply circuit on the host apparatus 1 side and thepower supply circuit 5 in the SSD 100. When an overcurrent is suppliedfrom an external power supply circuit, the fuse 8 is disconnected toprevent malfunction of the internal circuits.

The NAND memory 10 has four parallel operation elements 10 a to 10 dthat perform four parallel operations. One parallel operation elementhas two NAND memory packages. Each of the NAND memory packages includesa plurality of stacked NAND memory chips (e.g., 1 chip=2 GB). In thecase of FIG. 1, each of the NAND memory packages includes stacked fourNAND memory chips. The NAND memory 10 has a capacity of 64 GB. When eachof the NAND memory packages includes stacked eight NAND memory chips,the NAND memory 10 has a capacity of 128 GB.

The DRAM 20 functions as a cache for data transfer between the hostapparatus 1 and the NAND memory 10 and a memory for a work area. AnFeRAM (Ferroelectric Random Access Memory), PRAM (Phase-change RandomAccess Memory), or MRAM (Magnetoresistive Random Access Memory) can beused instead of the DRAM 20. The drive control circuit 4 performs datatransfer control between the host apparatus 1 and the NAND memory 10 viathe DRAM 20 and controls the respective components in the SSD 100. Thedrive control circuit 4 supplies a signal for status display to the LEDfor state display 6. The drive control circuit 4 also has a function ofreceiving a power-on reset signal from the power supply circuit 5 andsupplying a reset signal and a clock signal to respective units in theown circuit and the SSD 100.

Each of the NAND memory chips is configured by arraying a plurality ofphysical blocks as units of data erasing. FIG. 2( a) is a circuitdiagram of a configuration example of one physical block included in theNAND memory chip. Each physical block includes (p+1) NAND stringsarrayed in order along an X direction (p is an integer equal to orlarger than 0). A drain of a selection transistor ST1 included in eachof the (p+1) NAND strings is connected to bit lines BL0 to BLp and agate thereof is connected to a selection gate line SGD in common. Asource of a selection transistor ST2 is connected to a source line SL incommon and a gate thereof is connected to a selection gate line SGS incommon.

Each of memory cell transistors MT includes a MOSFET (Metal OxideSemiconductor Field Effect Transistor) including the stacked gatestructure formed on a semiconductor substrate. The stacked gatestructure includes a charge storage layer (a floating gate electrode)formed on the semiconductor substrate via a gate insulating film and acontrol gate electrode formed on the charge storage layer via aninter-gate insulating film. Threshold voltage changes according to thenumber of electrons accumulated in the floating gate electrode. Thememory cell transistor MT stores data according to a difference in thethreshold voltage. The memory cell transistor MT can be configured tostore one bit or can be configured to store multiple values (data equalto or larger than two bits).

The memory cell transistor MT is not limited to the structure having thefloating gate electrode and can be the structure such as a MONOS(Metal-Oxide-Nitride-Oxide-Silicon) type that can adjust a threshold bycausing a nitride film interface as a charge storage layer to trapelectrons. Similarly, the memory cell transistor MT of the MONOSstructure can be configured to store one bit or can be configured tostore multiple values (data equal to or larger than two bits).

In each of the NAND strings, (q+1) memory cell transistors MT arearranged between the source of the selection transistor ST1 and thedrain of the selection transistor ST2 such that current paths thereofare connected in series. In other words, the memory cell transistors MTare connected in series in a Y direction such that adjacent ones of thememory cell transistors MT share a diffusion region (a source region ora drain region).

Control gate electrodes of the memory cell transistors MT are connectedto word lines WL0 to WLq, respectively, in order from the memory celltransistor MT located on the most drain side. Therefore, a drain of thememory cell transistor MT connected to the word line WL0 is connected tothe source of the selection transistor ST1. A source of the memory celltransistor MT connected to the word line WLq is connected to the drainof the selection transistor ST2.

The word lines WL0 to WLq connect the control gate electrodes of thememory cell transistors MT in common among the NAND strings in thephysical block. In other words, the control gates of the memory celltransistors MT present in an identical row in the block are connected toan identical word line WL. (p+1) memory cell transistors MT connected tothe identical word line WL is treated as one page (physical page). Datawriting and data readout are performed by each physical page.

The bit lines BL0 to BLp connect drains of selection transistors ST1 incommon among the blocks. In other words, the NAND strings present in anidentical column in a plurality of blocks are connected to an identicalbit line BL.

FIG. 2( b) is a schematic diagram of a threshold distribution, forexample, in a quaternary data storage mode for storing two bits in onememory cell transistor MT. In the quaternary data storage mode, any oneof quaternary data “xy” defined by upper page data “x” and lower pagedata “y” can be stored in the memory cell transistor MT.

As the quaternary data “xy”, for example, “11”, “01”, “00”, and “10” areallocated in order of threshold voltages of the memory cell transistorMT. The data “11” is an erased state in which the threshold voltage ofthe memory cell transistor MT is negative.

In a lower page writing operation, the data “10” is selectively writtenin the memory cell transistor MT having the data “11” (in the erasedstate) according to the writing of the lower bit data “y”. A thresholddistribution of the data “10” before upper page writing is located aboutin the middle of threshold distributions of the data “01” and the data“00” after the upper page writing and can be broader than a thresholddistribution after the upper page writing. In a upper page writingoperation, writing of upper bit data “x” is selectively applied to amemory cell of the data “11” and a memory cell of the data “10”. Thedata “01” and the data “00” are written in the memory cells.

FIG. 3 is a block diagram of a hardware internal configuration exampleof the drive control circuit 4. The drive control circuit 4 includes adata access bus 101, a first circuit control bus 102, and a secondcircuit control bus 103. A processor 104 that controls the entire drivecontrol circuit 4 is connected to the first circuit control bus 102. Aboot ROM 105, in which a boot program for booting respective managementprograms (FW: firmware) stored in the NAND memory 10 is stored, isconnected to the first circuit control bus 102 via a ROM controller 106.A clock controller 107 that receives the power-on rest signal from thepower supply circuit 5 shown in FIG. 1 and supplies a reset signal and aclock signal to the respective units is connected to the first circuitcontrol bus 102.

The second circuit control bus 103 is connected to the first circuitcontrol bus 102. An I²C circuit 108 for receiving data from thetemperature sensor 7 shown in FIG. 1, a parallel IO (PIO) circuit 109that supplies a signal for status display to the LED for state display6, and a serial IO (SIO) circuit 110 that controls the RS232C I/F 3 areconnected to the second circuit control bus 103.

An ATA interface controller (ATA controller) 111, a first ECC (ErrorChecking and Correction) circuit 112, a NAND controller 113, and a DRAMcontroller 114 are connected to both the data access bus 101 and thefirst circuit control bus 102. The ATA controller 111 transmits data toand receives data from the host apparatus 1 via the ATA interface 2. AnSRAM 115 used as a data work area and a firm ware expansion area isconnected to the data access bus 101 via an SRAM controller 116. Whenthe firmware stored in the NAND memory 10 is started, the firmware istransferred to the SRAM 115 by the boot program stored in the boot ROM105.

The NAND controller 113 includes a NAND I/F 117 that performs interfaceprocessing for interface with the NAND memory 10, a second ECC circuit118, and a DMA controller for DMA transfer control 119 that performsaccess control between the NAND memory 10 and the DRAM 20. The secondECC circuit 118 performs encode of a second correction code and performsencode and decode of a first error correction code. The first ECCcircuit 112 performs decode of a second error correction code. The firsterror correction code and the second error correction code are, forexample, a hamming code, a BCH (Bose Chaudhuri Hocqenghem) code, an RS(Reed Solomon) code, or an LDPC (Low Density Parity Check) code.Correction ability of the second error correction code is higher thancorrection ability of the first error correction code.

As shown in FIGS. 1 and 3, in the NAND memory 10, the four paralleloperation elements 10 a to 10 d are connected in parallel to the NANDcontroller 112 in the drive control circuit 4 via four eight-bitchannels (4 ch). Three kinds of access modes explained below areprovided according to a combination of whether the four paralleloperation elements 10 a to 10 d are independently actuated or actuatedin parallel and whether a double speed mode (Multi Page Program/MultiPage Read/Multi Block Erase) provided in the NAND memory chip is used.

(1) 8-Bit Normal Mode

An 8-bit normal mode is a mode for actuating only one channel andperforming data transfer in 8-bit units. Writing and readout areperformed in the physical page size (4 kB). Erasing is performed in thephysical block size (512 kB). One logical block is associated with onephysical block and a logical block size is 512 kB.

(2) 32-Bit Normal Mode

A 32-bit normal mode is a mode for actuating four channels in paralleland performing data transfer in 32-bit units. Writing and readout areperformed in the physical page size×4 (16 kB). Erasing is performed inthe physical block size×4 (2 MB). One logical block is associated withfour physical blocks and a logical block size is 2 MB.

(3) 32-Bit Double Speed Mode

A 32-bit double speed mode is a mode for actuating four channels inparallel and performing writing and readout using a double speed mode ofthe NAND memory chip. Writing and readout are performed in the physicalpage size×4×2 (32 kB). Erasing is performed in the physical blocksize×4×2 (4 MB). One logical block is associated with eight physicalblocks and a logical block size is 4 MB.

In the 32-bit normal mode or the 32-bit double speed mode for actuatingfour channels in parallel, four or eight physical blocks operating inparallel are erasing units for the NAND memory 10 and four or eightphysical pages operating in parallel are writing units and readout unitsfor the NAND memory 10. In operations explained below, basically, the32-bit double speed mode is used. For example, it is assumed that onelogical block=4 MB=2^(i) tracks=2^(j) pages=2^(k) clusters=2^(l) sectors(i, j, k, and l are natural numbers and a relation of i<j<k<l holds).

A logical block accessed in the 32-bit double speed mode is accessed in4 MB units. Eight (2×4 ch) physical blocks (one physical block=512 kB)are associated with the logical block. When the bad block BB managed inphysical block units is detected, the bad block BB is unusable.Therefore, in such a case, a combination of the eight physical blocksassociated with the logical block is changed to not include the badblock BB.

FIG. 4 is a block diagram of a functional configuration example offirmware realized by the processor 104. Functions of the firmwarerealized by the processor 104 are roughly classified into a datamanaging unit 120, an ATA-command processing unit 121, a securitymanaging unit 122, a boot loader 123, an initialization managing unit124, and a debug supporting unit 125.

The data managing unit 120 controls data transfer between the NANDmemory 10 and the DRAM 20 and various functions concerning the NANDmemory 10 via the NAND controller 112 and the first ECC circuit 114. TheATA-command processing unit 121 performs data transfer processingbetween the DRAM 20 and the host apparatus 1 in cooperation with thedata managing unit 120 via the ATA controller 110 and the DRAMcontroller 113. The security managing unit 122 manages various kinds ofsecurity information in cooperation with the data managing unit 120 andthe ATA-command processing unit 121.

The boot loader 123 loads, when a power supply is turned on, themanagement programs (firmware) from the NAND memory 10 to the SRAM 120.The initialization managing unit 124 performs initialization ofrespective controllers and circuits in the drive control circuit 4. Thedebug supporting unit 125 processes data for debug supplied from theoutside via the RS232C interface. The data managing unit 120, theATA-command processing unit 121, and the security managing unit 122 aremainly functional units realized by the processor 104 executing themanagement programs stored in the SRAM 114.

In this embodiment, functions realized by the data managing unit 120 aremainly explained. The data managing unit 120 performs, for example,provision of functions that the ATA-command processing unit 121 requeststhe NAND memory 10 and the DRAM 20 as storage devices to provide (inresponse to various commands such as a Write request, a Cache Flushrequest, and a Read request from the host apparatus), management of acorrespondence relation between a host address region and the NANDmemory 10 and protection of management information, provision of fastand highly efficient data readout and writing functions using the DRAM20 and the NAND 10, ensuring of reliability of the NAND memory 10.

FIG. 5 is a diagram of functional blocks formed in the NAND memory 10and the DRAM 20. A write cache (WC) 21 and a read cache (RC) 22configured on the DRAM 20 are interposed between the host 1 and the NANDmemory 10. The WC 21 temporarily stores Write data from the hostapparatus 1. The RC 22 temporarily stores Read data from the NAND memory10. The WC 21 and the RC 22 may be configured on different DRAM chips orother kind of memory chips described above.

The logical blocks in the NAND memory 10 are allocated to respectivemanagement areas of a pre-stage storage area (FS: Front Storage) 12, anintermediate stage storage area (IS: Intermediate Storage) 13, and amain storage area (MS: Main Storage) 11 by the data managing unit 120 inorder to reduce an amount of erasing for the NAND memory 10 duringwriting. The FS 12 manages data from the WC 21 in cluster units, i.e.,“small units” and stores small data (fine grained data) for a shortperiod. The IS 13 manages data overflowing from the FS 12 in clusterunits, i.e., “small units” and stores small data (fine grained data) fora long period. The MS 11 stores data from the WC 21, the FS 12, and theIS 13 in track units, i.e., “large units” and stores large data (coarsegrained data) for a long period. For example, storage capacities are ina relation of MS>IS and FS>WC.

When the small management unit is applied to all the storage areas ofthe NAND memory 10, a size of a management table explained later isenlarged and does not fit in the DRAM 20. Therefore, the respectivestorages of the NAND memory 10 are configured to manage, in smallmanagement units, only data just written recently and small data withlow efficiency of writing in the NAND memory 10. The techniques usingthe “small units” together with the “large units” in the SSD 100 aredescribed in the International Application No. PCT2008/JP/073950, theentire contents of which are incorporated herein by reference.

FIG. 6 is a more detailed functional block diagram related to writeprocessing from the WC 21 to the NAND memory 10. An FS input buffer(FSIB) 12 a that buffers data from the WC 21 is provided at a pre-stageof the FS 12. An MS input buffer (MSIB) 11 a that buffers data from theWC 21, the FS 12, or the IS 13 is provided at a pre-stage of the MS 11.A track pre-stage storage area (TFS) 11 b is provided in the MS 11. TheTFS 11 b is a buffer that has the FIFO (First in First out) structureinterposed between the MSIB 11 a and the MS 11. Data recorded in the TFS11 b is data with an update frequency higher than that of data recordedin the MS 11. Any of the logical blocks in the NAND memory 10 isallocated to the MS 11, the MSIB 11 a, the TFS 11 b, the FS 12, the FSIB12 a, and the IS 13.

Specific functional configurations of the respective components shown inFIGS. 5 and 6 are explained in detail. When the host apparatus 1performs Read or Write for the SSD 100, the host apparatus 1 inputs LBA(Logical Block Addressing) as a logical address via the ATA interface.As shown in FIG. 7, the LBA is a logical address in which serial numbersfrom 0 are attached to sectors (size: 512 B). In this embodiment, asmanagement units for the WC 21, the RC 22, the FS 12, the IS 13, and theMS 11, which are the components shown in FIG. 5, a logical clusteraddress formed of a bit string equal to or higher in order than alow-order (l−k+1)th bit of the LBA and a logical track address formed ofbit strings equal to or higher in order than a low-order (l−i+1)th bitof the LBA are defined. One cluster=2^((l−k)) sectors and onetrack=2^((k−i)) clusters.

Read Cache (RC) 22

The RC 22 is explained. The RC 22 is an area for temporarily storing, inresponse to a Read request from the ATA-command processing unit 121,Read data from the NAND memory 10 (the FS 12, the IS 13, and the MS 11).In this embodiment, the RC 22 is managed in, for example, anm-line/n-way (m is a natural number equal to or larger than 2^((k−i))and n is a natural number equal to or larger than 2) set associativesystem and can store data for one cluster in one entry. A line isdetermined by LSB (k−i) bits of the logical cluster address. The RC 22can be managed in a full-associative system or can be managed in asimple FIFO system.

Write Cache (WC) 21

The WC 21 is explained. The WC 21 is an area for temporarily storing, inresponse to a Write request from the ATA-command processing unit 121,Write data from the host apparatus 1. The WC 21 is managed in them-line/n-way (m is a natural number equal to or larger than 2^((k−i))and n is a natural number equal to or larger than 2) set associativesystem and can store data for one cluster in one entry. A line isdetermined by LSB (k−i) bits of the logical cluster address. Forexample, a writable way is searched in order from a way 1 to a way n.Tracks registered in the WC 21 are managed in LRU (Least Recently Used)by the FIFO structure of a WC track management table 24 explained latersuch that the order of earliest update is known. The WC 21 can bemanaged by the full-associative system. The WC 21 can be different fromthe RC 22 in the number of lines and the number of ways.

Data written according to the Write request is once stored on the WC 21.A method of determining data to be flushed from the WC 21 to the NAND 10complies with rules explained below.

(i) When a writable way in a line determined by a tag is a last (in thisembodiment, nth) free way, i.e., when the last free way is used, a trackupdated earliest based on an LRU among tracks registered in the line isdecided to be flushed.

(ii) When the number of different tracks registered in the WC 21 exceedsa predetermined permissible number, tracks with the numbers of clusterssmaller than a predetermined number in a WC are decided to be flushed inorder of LRUs.

Tracks to be flushed are determined according to the policies explainedabove. In flushing the tracks, all data included in an identical trackis flushed. When an amount of data to be flushed exceeds, for example,50% of a track size, the data is flushed to the MS 11. When an amount ofdata to be flushed does not exceed, for example, 50% of a track size,the data is flushed to the FS 12.

When track flush is performed under the condition (i) and the data isflushed to the MS 11, a track satisfying a condition that an amount ofdata to be flushed exceeds 50% of a track size among the tracks in theWC 21 is selected and added to flush candidates according to the policy(i) until the number of tracks to be flushed reaches 2^(i) (when thenumber of tracks is equal to or larger than 2^(i) from the beginning,until the number of tracks reaches 2^(i+1)). In other words, when thenumber of tracks to be flushed is smaller than 2^(i), tracks havingvalid clusters more than 2^((k−i−l)) are selected in order from theoldest track in the WC and added to the flush candidates until thenumber of tracks reaches 2^(i).

When track flush is performed under the condition (i) and the track isflushed to the FS 12, a track satisfying the condition that an amount ofdata to be flushed does not exceed 50% of a track size is selected inorder of LRUs among the tracks in the WC 21 and clusters of the trackare added to the flush candidates until the number of clusters to beflushed reaches 2^(k). In other words, clusters are extracted fromtracks having 2^((k−i−l)) or less valid clusters by tracing the tracksin the WC in order from the oldest one and, when the number of validclusters reaches 2^(k), the clusters are flushed to the FSIB 12 a inlogical block units. However, when 2^(k) valid clusters are not found,clusters are flushed to the FSIB 12 a in logical page units. A thresholdof the number of valid clusters for determining whether the flush to theFS 12 is performed in logical block units or logical page units is notlimited to a value for one logical block, i.e., 2^(k) and can be a valueslightly smaller than the value for one logical block.

In a Cache Flush request from the ATA-command processing unit 121, allcontents of the WC 21 are flushed to the FS 12 or the MS 11 underconditions same as the above (when an amount of data to be flushedexceeds 50% of a track size, the data is flushed to the MS 11 and, whenthe amount of data does not exceed 50%, the data is flushed to the FS12).

Pre-Stage Storage Area (FS) 12

The FS 12 is explained. The FS 12 adapts an FIFO structure of logicalblock units in which data is managed in cluster units. The FS 12 is abuffer for regarding that data passing through the FS 12 has an updatefrequency higher than that of the IS 13 at the post stage. In otherwords, in the FIFO structure of the FS 12, a valid cluster (a latestcluster) passing through the FIFO is invalidated when rewriting in thesame address from the host is performed. Therefore, the cluster passingthrough the FS 12 can be regarded as having an update frequency higherthan that of a cluster flushed from the FS 12 to the IS 13 or the MS 11.

By providing the FS 12, likelihood of mixing of data with a high updatefrequency in compaction processing in the IS 13 at the post stage isreduced. When the number of valid clusters of a logical block is reducedto 0 by the invalidation, the logical block is released and allocated tothe free block FB. When the logical block in the FS 12 is invalidated, anew free block FB is acquired and allocated to the FS 12.

When cluster flush from the WC 21 to the FS 12 is performed, the clusteris written in a logical block allocated to the FSIB 12 a. When logicalblocks, for which writing of all logical pages is completed, are presentin the FSIB 12 a, the logical blocks are moved from the FSIB 12 a to theFS 12 by CIB processing explained later. In moving the logical blocksfrom the FSIB 12 a to the FS 12, when the number of logical blocks ofthe FS 12 exceeds a predetermined upper limit value allowed for the FS12, an oldest logical block is flushed from the FS 12 to the IS 13 orthe MS 11. For example, a track with a ratio of valid clusters in thetrack equal to or larger than 50% is written in the MS 11 (the TFS 11 b)and a logical block in which the valid cluster remains is moved to theIS 13.

As the data movement between components in the NAND memory 10, there aretwo ways, i.e., Move and Copy. Move is a method of simply performingrelocation of a pointer of a management table explained later and notperforming actual rewriting of data. Copy is a method of actuallyrewriting data stored in one component to the other component in pageunits, track units, or block units.

Intermediate Stage Storage Area (IS) 13

The IS 13 is explained. In the IS 13, management of data is performed incluster units in the same manner as the FS 12. Data stored in the IS 13can be regarded as data with a low update frequency. When movement(Move) of a logical block from the FS 12 to the IS 13, i.e., flush ofthe logical block from the FS 12 is performed, a logical block as anflush object, which is previously a management object of the FS 12, ischanged to a management object of the IS 13 by the relocation of thepointer. According to the movement of the logical block from the FS 12to the IS 13, when the number of blocks of the IS 13 exceeds apredetermined upper limit value allowed for the IS 13, i.e., when thenumber of writable free blocks FB in the IS decreases to be smaller thana threshold, data flush from the IS 13 to the MS 11 and compactionprocessing are executed. The number of blocks of the IS 13 is returnedto a specified value.

The IS 13 executes flush processing and compaction processing explainedbelow using the number of valid clusters in a track.

Tracks are sorted in order of the number of valid clusters×valid clustercoefficient (the number weighted according to whether a track is presentin a logical block in which an invalid track is present in the MS 11;the number is larger when the invalid track is present than when theinvalid track is not present). 2^(i+l) tracks (for two logical blocks)with a large value of a product are collected, increased to be naturalnumber times as large as a logical block size, and flushed to the MSIB11 a.

When a total number of valid clusters of two logical blocks with asmallest number of valid clusters is, for example, equal to or largerthan 2^(k) (for one logical block), which is a predetermined set value,the step explained above is repeated (to perform the step until a freeblock FB can be created from two logical blocks in the IS).

2^(k) clusters are collected in order from logical blocks with asmallest number of valid clusters and compaction is performed in the IS.

Here, the two logical blocks with the smallest number of valid clustersare selected. However, the number is not limited to two and only has tobe a number equal to or larger than two. The predetermined set valueonly has to be equal to or smaller than the number of clusters that canbe stored in the number of logical blocks smaller than the number ofselected logical blocks by one.

Main Storage Area (MS) 11

The MS 11 is explained. In the MS 11, management of data is performed intrack units. Data stored in the MS 11 can be regarded as having a lowupdate frequency. When Copy or Move of track from the WC 21, the FS 12,or the IS 13 to the MS 11 is performed, the track is written in alogical block allocated to the MSIB 11 a. On the other hand, when onlydata (clusters) in a part of the track is written from the FS 12, the IS13 or the like, passive merge explained later for merging existing trackin the MS 11 and new data to create new track and, then, writing thecreated track in the MSIB 11 a is performed. When invalid tracks areaccumulated in the MS 11 and the number of logical blocks allocated tothe MS 11 exceeds the upper limit of the number of blocks allowed forthe MS 11, compaction processing is performed to create a free block FB.

As the compaction processing of the MS 11, for example, a methodexplained below with attention paid to only the number of valid tracksin a logical block is carried out.

Logical blocks are selected from one with a smallest number of validtracks until a free block FB can be created by combining invalid tracks.

Compaction is executed while passive merge for integrating tracks storedin the selected logical blocks with data in the WC 21, the FS 12, or theIS 13 is performed.

A logical block in which 2^(i) tracks can be integrated is output to theTFS 11 b (2^(i) track MS compaction) and tracks smaller in number than2^(i) are output to the MSIB 11 a (less than 2^(i) track compaction) tocreate a larger number of free blocks FB.

The TFS 11 b adapts an FIFO structure of logical block units in whichdata is managed in track units. The TFS 11 b is a buffer for regardingthat data passing through the TFS 11 b has an update frequency higherthan that of the MS 11 at the post stage. In other words, in the FIFOstructure of the TFS 11 b, a valid track (a latest track) passingthrough the FIFO is invalidated when rewriting in the same address fromthe host is performed. Therefore, a track passing through the TFS 11 bcan be regarded as having an update frequency higher than that of atrack flushed from the TFS 11 b to the MS 11.

FIG. 8 is a diagram of a management table for the data managing unit 120to control and manage the respective components shown in FIGS. 5 and 6.The data managing unit 120 has, as explained above, the function ofbridging the ATA-command processing unit 121 and the NAND memory 10 andincludes a DRAM-layer managing unit 120 a that performs management ofdata stored in the DRAM 20, a logical-NAND-layer managing unit 120 bthat performs management of data stored in the NAND memory 10, and aphysical-NAND-layer managing unit 120 c that manages the NAND memory 10as a physical storage device. An RC cluster management table 23, a WCtrack management table 24, and a WC cluster management table 25 arecontrolled by the DRAM-layer managing unit 120 a. A track managementtable 30, an FS/IS management table 40, an MS logical block managementtable 35, an FS/IS logical block management table 42, and an intra-FS/IScluster management table 44 are managed by the logical-NAND-layermanaging unit 120 b. A logical-to-physical translation table 50 ismanaged by the physical-NAND-layer managing unit 120 c.

The RC 22 is managed by the RC cluster management table 23, which is areverse lookup table. In the reverse lookup table, from a position of astorage device, a logical address stored in the position can besearched. The WC 21 is managed by the WC cluster management table 25,which is a reverse lookup table, and the WC track management table 24,which is a forward lookup table. In the forward lookup table, from alogical address, position of a storage device in which datacorresponding to the logical address is present can be searched.

Logical addresses of the FS 12 (the FSIB 12 a), the IS 13, and the MS 11(the TFS 11 b and the MSIB 11 a) in the NAND memory 10 are managed bythe track management table 30, the FS/IS management table 40, the MSlogical block management table 35, the FS/IS logical block managementtable 42, and the intra-FS/IS cluster management table 44. In the FS 12(the FSIB 12 a), the IS 13, and the MS 11 (the TFS 11 b and MSIB 11 a)in the NAND memory 10, conversion of a logical address and a physicaladdress is performed of the logical-to-physical translation table 50.These management tables are stored in an area on the NAND memory 10 andread onto the DRAM 20 from the NAND memory 10 during initialization ofthe SSD 100.

RC Cluster Management Table 23 (Reverse Lookup)

The RC cluster management table 23 is explained with reference to FIG.9. As explained above, the RC 22 is managed in the n-way set associativesystem indexed by logical cluster address LSB (k−i) bits. The RC clustermanagement table 23 is a table for managing tags of respective entriesof the RC (the cluster size×m-line×n-way) 22. Each of the tags includesa state flag 23 a including a plurality of bits and a logical trackaddress 23 b. The state flag 23 a includes, besides a valid bitindicating whether the entry may be used (valid/invalid), for example, abit indicating whether the entry is on a wait for readout from the NANDmemory 10 and a bit indicating whether the entry is on a wait forreadout to the ATA-command processing unit 121. The RC clustermanagement table 23 functions as a reverse lookup table for searchingfor a logical track address coinciding with LBA from a tag storageposition on the DRAM 20.

WC Cluster Management Table 25 (Reverse Lookup)

The WC cluster management table 25 is explained with reference to FIG.10. As explained above, the WC 21 is managed in the n-way setassociative system indexed by logical cluster address LSB (k−i) bits.The WC cluster management table 25 is a table for managing tags ofrespective entries of the WC (the cluster size×m-line×n-way) 21. Each ofthe tags includes a state flag 25 a of a plurality of bits, a sectorposition bitmap 25 b, and a logical track address 25 c.

The state flag 25 a includes, besides a valid bit indicating whether theentry may be used (valid/invalid), for example, a bit indicating whetherthe entry is on a wait for flush to the NAND memory 10 and a bitindicating whether the entry is on a wait for writing from theATA-command processing unit 121. The sector position bitmap 25 bindicates which of 2^((l−k)) sectors included in one cluster storesvalid data by expanding the sectors into 2^((l−k)) bits. With the sectorposition bitmap 25 b, management in sector units same as the LBA can beperformed in the WC 21. The WC cluster management table 25 functions asa reverse lookup table for searching for a logical track addresscoinciding with the LBA from a tag storage position on the DRAM 20.

WC Track Management Table 24 (Forward Lookup)

The WC track management table 24 is explained with reference to FIG. 11.The WC track management table 24 is a table for managing information inwhich clusters stored on the WC 21 are collected in track units andrepresents the order (LRU) of registration in the WC 21 among the tracksusing the linked list structure having an FIFO-like function. The LRUcan be represented by the order updated last in the WC 21. An entry ofeach list includes a logical track address 24 a, the number of validclusters 24 b in the WC 21 included in the logical track address, away-line bitmap 24 c, and a next pointer 24 d indicating a pointer tothe next entry. The WC track management table 24 functions as a forwardlookup table because required information is obtained from the logicaltrack address 24 a.

The way-line bitmap 24 c is map information indicating in which of m×nentries in the WC 21 a valid cluster included in the logical trackaddress in the WC 21 is stored. The Valid bit is “1” in an entry inwhich the valid cluster is stored. The way-line bitmap 24 c includes,for example, (one bit (valid)+log₂n bits (n-way))×m bits (m-line). TheWC track management table 24 has the linked list structure. Onlyinformation concerning the logical track address present in the WC 21 isentered.

Track Management Table 30 (Forward Lookup)

The track management table 30 is explained with reference to FIG. 12.The track management table 30 is a table for managing a logical dataposition on the MS 11 in logical track address units. When data isstored in the FS 12 or the IS 13 in cluster units, the track managementtable 30 stores basic information concerning the data and a pointer todetailed information. The track management table 30 is configured in anarray format having a logical track address 30 a as an index. Each entryhaving the logical track address 30 a as an index includes informationsuch as a cluster bitmap 30 b, a logical block ID 30 c+an intra-logicalblock track position 30 d, a cluster table pointer 30 e, the number ofFS clusters 30 f, and the number of IS clusters 30 g. The trackmanagement table 30 functions as a forward lookup table because, using alogical track address as an index, required information such as alogical block ID (corresponding to a storage device position) in whichtrack corresponding to the logical track address is stored.

The cluster bitmap 30 b is a bitmap obtained by dividing 2^((k−i))clusters belonging to one logical track address range into, for example,eight in ascending order of logical cluster addresses. Each of eightbits indicates whether clusters corresponding to 2^((k−i−3)) clusteraddresses are present in the MS 11 or present in the FS 12 or the IS 13.When the bit is “0”, this indicates that the clusters as search objectsare surely present in the MS 11. When the bit is “1”, this indicatesthat the clusters are likely to be present in the FS 12 or the IS 13.

The logical block ID 30 c is information for identifying a logical blockID in which track corresponding to the logical track address is stored.The intra-logical block track position 30 d indicates a storage positionof a track corresponding to the logical track address (30 a) in thelogical block designated by the logical block ID 30 c. Because onelogical block includes maximum 2^(i) valid tracks, the intra-logicalblock track position 30 d identifies 2^(i) track positions using i bits.

The cluster table pointer 30 e is a pointer to a top entry of each listof the FS/IS management table 40 having the linked list structure. Inthe search through the cluster bitmap 30 b, when it is indicated thatthe cluster is likely to be present in the FS 12 or the IS 13, searchthrough the FS/IS management table 40 is executed by using the clustertable pointer 30 e. The number of FS clusters 30 f indicates the numberof valid clusters present in the FS 12. The number of IS clusters 30 gindicates the number of valid clusters present in the IS 13.

FS/IS Management Table 40 (Forward Lookup)

The FS/IS management table 40 is explained with reference to FIG. 13.The FS/IS management table 40 is a table for managing a position of datastored in the FS 12 (including the FSIB 12 a) or the IS 13 in logicalcluster addresses. As shown in FIG. 13, the FS/IS management table 40 isformed in an independent linked list format for each logical trackaddress. As explained above, a pointer to a top entry of each list isstored in a field of the cluster table pointer 30 e of the trackmanagement table 30. In FIG. 13, linked lists for two logical trackaddresses are shown. Each entry includes a logical cluster address 40 a,a logical block ID 40 b, an intra-logical block cluster position 40 c,an FS/IS block ID 40 d, and a next pointer 40 e. The FS/IS managementtable 40 functions as a forward lookup table because requiredinformation such as the logical block ID 40 b and the intra-logicalblock cluster position 40 c (corresponding to a storage device position)in which cluster corresponding to the logical cluster address 40 a isstored is obtained from the logical cluster address 40 a.

The logical block ID 40 b is information for identifying a logical blockID in which cluster corresponding to the logical cluster address 40 a isstored. The intra-logical block cluster position 40 c indicates astorage position of a cluster corresponding to the logical clusteraddress 40 a in a logical block designated by the logical block ID 40 b.Because one logical block includes maximum 2^(k) valid clusters, theintra-logical block cluster position 40 c identifies 2^(k) positionsusing k bits. An FS/IS block ID, which is an index of the FS/IS logicalblock management table 42 explained later, is registered in the FS/ISblock ID 40 d. The FS/IS block ID 40 d is information for identifying alogical block belonging to the FS 12 or the IS 13. The FS/IS block ID 40d in the FS/IS management table 40 is registered for link to the FS/ISlogical block management table 42 explained later. The next pointer 40 eindicates a pointer to the next entry in the same list linked for eachlogical track address.

MS Logical Block Management Table 35 (Reverse Lookup)

The MS logical block management table 35 is explained with reference toFIG. 14. The MS logical block management table 35 is a table forunitarily managing information concerning a logical block used in the MS11 (e.g., which track is stored and whether a track position isadditionally recordable). In the MS logical block management table 35,information concerning logical blocks belonging to the FS 12 (includingthe FSIB 12) and the IS 13 is also registered. The MS logical blockmanagement table 35 is formed in an array format having a logical blockID 35 a as an index. The number of entries can be 32 K entries at themaximum in the case of the 128 GB NAND memory 10. Each of the entriesincludes a track management pointer 35 b for 2^(i) tracks, the number ofvalid tracks 35 c, a writable top track 35 d, and a valid flag 35 e. TheMS logical block management table 35 functions as a reverse lookup tablebecause required information such as a logical track address stored inthe logical block is obtained from the logical block ID 35 acorresponding to a storage device position.

The track management pointer 35 b stores a logical track addresscorresponding to each of 2^(i) track positions in the logical blockdesignated by the logical block ID 35 a. It is possible to searchthrough the track management table 30 having the logical track addressas an index using the logical track address. The number of valid tracks35 c indicates the number of valid tracks (maximum 2^(i)) among tracksstored in the logical block designated by the logical block ID 35 a. Thewritable top track position 35 d indicates a top position (0 to 2^(i−l),2^(i) when additional recording is finished) additionally recordablewhen the logical block designated by the logical block ID 35 a is ablock being additionally recorded. The valid flag 35 e is “1” when thelogical block entry is managed as the MS 11 (including the MSIB 11 a).Here, “additional recording” means that writing cluster or track, inappending manner, to empty logical pages in a logical block.

FS/IS Logical Block Management Table 42 (Reverse Lookup)

The FS/IS logical block management table 42 is explained with referenceto FIG. 15. The FS/IS logical block management table 42 is formed in anarray format having an FS/IS block ID 42 a as an index. The FS/ISlogical block management table 42 is a table for managing informationconcerning a logical block used as the FS 12 or the IS 13(correspondence to a logical block ID, an index to the intra-FS/IScluster management table 44, whether the logical block is additionallyrecordable, etc.). The FS/IS logical block management table 42 isaccessed by mainly using the FS/IS block ID 40 d in the FS/IS managementtable 40. Each entry includes a logical block ID 42 b, an intra-blockcluster table 42 c, the number of valid clusters 42 d, a writable toppage 42 e, and a valid flag 42 f. The MS logical block management table35 functions as a reverse lookup table because required information suchas cluster stored in the logical block is obtained from the FS/IS blockID 42 corresponding to a storage device position.

Logical block IDs corresponding to logical blocks belonging to the FS 12(including the FSIB 12) and the IS 13 among logical blocks registered inthe MS logical block management table 35 are registered in the logicalblock ID 42 b. An index to the intra-FS/IS cluster management table 44explained later indicating a logical cluster designated by which logicalcluster address is registered in each cluster position in a logicalblock is registered in the intra-block cluster table 42 c. The number ofvalid clusters 42 d indicates the number of (maximum 2^(k)) validclusters among clusters stored in the logical block designated by theFS/IS block ID 42 a. The writable top page position 42 e indicates a toppage position (0 to 2^(i) when additional recording is finished)additionally recordable when the logical block designated by the FS/ISblock ID 42 a is a block being additionally recorded. The valid flag 42f is “1” when the logical block entry is managed as the FS 12 (includingthe FSIB 12) or the IS 13.

Intra-FS/IS Cluster Management Table 44 (Reverse Lookup)

The intra-FS/IS cluster management table 44 is explained with referenceto FIG. 16. The intra-FS/IS cluster management table 44 is a tableindicating which cluster is recorded in each cluster position in alogical block used as the FS 12 or the IS 13. The intra-FS/IS clustermanagement table 44 has 2^(j) pages×2^((k−j)) clusters=2^(k) entries perone logical block. Information corresponding to 0th to 2^(k)-1th clusterpositions among cluster positions in the logical block is arranged incontinuous areas. Tables including the 2^(k) pieces of information arestored by the number equivalent to the number of logical blocks (P)belonging to the FS 12 and the IS 13. The intra-block cluster table 42 cof the FS/IS logical block management table 42 is positional information(a pointer) for the P tables. A position of each entry 44 a arranged inthe continuous areas indicates a cluster position in one logical block.As content of the entry 44 a, a pointer to a list including a logicalcluster address managed by the FS/IS management table 40 is registeredsuch that it is possible to identify which cluster is stored in thecluster position. In other words, the entry 44 a does not indicate thetop of a linked list. A pointer to one list including the logicalcluster address in the linked list is registered in the entry 44 a.

Logical-to-Physical Translation Table 50 (Forward Lookup)

The logical-to-physical translation table 50 is explained with referenceto FIG. 17. The logical-to-physical translation table 50 is formed in anarray format having a logical block ID 50 a as an index. The number ofentries can be maximum 32 K entries in the case of the 128 GB NANDmemory 10. The logical-to-physical translation table 50 is a table formanaging information concerning conversion between a logical block IDand a physical block ID and the life. Each of the entries includes aphysical block address 50 b, the number of times of erasing 50 c, andthe number of times of readout 50 d. The logical-to-physical translationtable 50 functions as a forward lookup table because requiredinformation such as a physical block ID (a physical block address) isobtained from a logical block ID.

The physical block address 50 b indicates eight physical block IDs(physical block addresses) belonging to one logical block ID 50 a. Thenumber of times of erasing 50 c indicates the number of times of erasingof the logical block ID. A bad block (BB) is managed in physical block(512 KB) units. However, the number of times of erasing is managed inone logical block (4 MB) units in the 32-bit double speed mode. Thenumber of times of readout 50 d indicates the number of times of readoutof the logical block ID. The number of times of erasing 50 c can be usedin, for example, wear leveling processing for leveling the number oftimes of rewriting of a NAND-type flash memory. The number of times ofreadout 50 d can be used in refresh processing for rewriting data storedin a physical block having deteriorated retention properties.

An example of the wear leveling processing is described in theInternational Application No. PCT/JP2008/066508 and No.PCT/JP2008/066507. An example of the refresh processing is described inthe International Application No. PCT/JP2008/067597, the entire contentsof which are incorporated herein by reference.

The management tables shown in FIG. 8 are collated by management objectas explained below.

RC management: The RC cluster management table 23

WC management: The WC cluster management table 25 and the WC trackmanagement table 24

MS management: The track management table 30 and the MS logical blockmanagement table 35

FS/IS management: The track management table 30, the FS/IS managementtable 40, the MS logical block management table 35, the FS/IS logicalblock management table 42, and the intra-FS/IS cluster management table44

The structure of an MS area including the MS 11, the MSIB 11 a, and theTFS 11 b is managed in an MS structure management table (not shown).Specifically, logical blocks and the like allocated to the MS 11, theMSIB 11 a, and the TFS 11 b are managed. The structure of an FS/IS areaincluding the FS 12, the FSIB 12 a, and the IS 13 is managed in an FS/ISstructure management table (not shown). Specifically, logical blocks andthe like allocated to the FS 12, the FSIB 12 a, and the IS 13 aremanaged.

Read Processing

Read processing is explained with reference to a flowchart shown in FIG.18. When a Read command, LBA as a readout address, and a readout sizeare input from the ATA-command processing unit 121, the data managingunit 120 searches through the RC cluster management table 23 shown inFIG. 9 and the WC cluster management table 25 shown in FIG. 10 (stepS100). Specifically, the data managing unit 120 selects linescorresponding to LSB (k−i) bits (see FIG. 7) of a logical clusteraddress of the LBA from the RC cluster management table 23 and the WCcluster management table 25 and compares logical track addresses 23 band 25 c entered in each way of the selected lines with a logical trackaddress of the LBA (step S110). When a way such that a logical trackaddress entered in itself coincides with a logical track address of LBAis present, the data managing unit 120 regards this as cache hit. Thedata managing unit 120 reads out data of the WC 21 or the RC 22corresponding to the hit line and way of the RC cluster management table23 or the WC cluster management table 25 and sends the data to theATA-command processing unit 121 (step S115).

When there is no hit in the RC 22 or the WC 21 (step S110), the datamanaging unit 120 searches in which part of the NAND memory 10 a clusteras a search object is stored. First, the data managing unit 120 searchesthrough the track management table 30 shown in FIG. 12 (step S120). Thetrack management table 30 is indexed by the logical track address 30 a.Therefore, the data managing unit 120 checks only entries of the logicaltrack address 30 a coinciding with the logical track address designatedby the LBA.

The data managing unit 120 selects a corresponding bit from the clusterbitmap 30 b based on a logical cluster address of the LBA desired to bechecked. When the corresponding bit indicates “0”, this means thatlatest data of the cluster is surely present the MS (step S130). In thiscase, the data managing unit 120 obtains logical block ID and a trackposition in which the track is present from the logical block ID 30 cand the intra-logical block track position 30 d in the same entry of thelogical track address 30 a. The data managing unit 120 calculates anoffset from the track position using LSB (k−i) bits of the logicalcluster address of the LBA. Consequently, the data managing unit 120 cancalculate position where cluster corresponding to the logical clusteraddress in the NAND memory 10 is stored. Specifically, thelogical-NAND-layer managing unit 120 b gives the logical block ID 30 cand the intra-logical block position 30 d acquired from the trackmanagement table 30 as explained above and the LSB (k−i) bits of thelogical cluster address of the LBA to the physical-NAND-layer managingunit 120 c.

The physical-NAND-layer managing unit 120 c acquires a physical blockaddress (a physical block ID) corresponding to the logical block ID 30 cfrom the logical-to-physical translation table 50 shown in FIG. 17having the logical block ID as an index (step S160). The data managingunit 120 calculates a track position (a track top position) in theacquired physical block ID from the intra-logical block track position30 d and further calculates, from the LSB (k−i) bits of the logicalcluster address of the LBA, an offset from the calculated track topposition in the physical block ID. Consequently, the data managing unit120 can acquire cluster in the physical block. The data managing unit120 sends the cluster acquired from the MS 11 of the NAND memory 10 tothe ATA-command processing unit 121 via the RC 22 (step S180).

On the other hand, when the corresponding bit indicates “1” in thesearch through the cluster bitmap 30 b based on the logical clusteraddress of the LBA, it is likely that the cluster is stored in the FS 12or the IS 13 (step S130). In this case, the data managing unit 120extracts an entry of the cluster table pointer 30 e among relevantentries of the logical track address 30 a in the track management table30 and sequentially searches through linked lists corresponding to arelevant logical track address of the FS/IS management table 40 usingthis pointer (step S140). Specifically, the data managing unit 120searches for an entry of the logical cluster address 40 a coincidingwith the logical cluster address of the LBA in the linked list of therelevant logical track address. When the coinciding entry of the logicalcluster address 40 a is present (step S150), the data managing unit 120acquires the logical block ID 40 b and the intra-logical block clusterposition 40 c in the coinciding list. In the same manner as explainedabove, the data managing unit 120 acquires the cluster in the physicalblock using the logical-to-physical translation table 50 (steps S160 andS180). Specifically, the data managing unit 120 acquires physical blockaddresses (physical block IDs) corresponding to the acquired logicalblock ID from the logical-to-physical translation table 50 (step S160)and calculates a cluster position of the acquired physical block ID froman intra-logical block cluster position acquired from an entry of theintra-logical block cluster position 40 c. Consequently, the datamanaging unit 120 can acquire the cluster in the physical block. Thedata managing unit 120 sends the cluster acquired from the FS 12 or theIS 13 of the NAND memory 10 to the ATA-command processing unit 121 viathe RC 22 (step S180).

When the cluster as the search object is not present in the searchthrough the FS/IS management table 40 (step S150), the data managingunit 120 searches through the entries of the track management table 30again and decides a position on the MS 11 (step S170).

Write Processing

Write processing is explained with reference to a flowchart shown inFIG. 19. Data written by a Write command is always once stored on the WC21. Thereafter, the data is written in the NAND memory 10 according toconditions. In the write processing, it is likely that flush processingand compaction processing are performed. In this embodiment, the writeprocessing is roughly divided into two stages of write cache flashprocessing (hereinafter, WCF processing) and clean input bufferprocessing (hereinafter, CIB processing). Steps S300 to S320 indicateprocessing from a Write request from the ATA-command processing unit 121to the WCF processing. Step S330 to the last step indicate the CIBprocessing.

The WCF processing is processing for copying data in the WC 21 to theNAND memory 10 (the FSIB 12 a of the FS 12 or the MSIB 11 a of the MS11). A Write request or a Cache Flush request alone from the ATA-commandprocessing unit 121 can be completed only by this processing. This makesit possible to limit a delay in the started processing of the Writerequest of the ATA-command processing unit 121 to, at the maximum, timefor writing in the NAND memory 10 equivalent to a capacity of the WC 21.

The CIB processing includes processing for moving the data in the FSIB12 a written by the WCF processing to the FS 12 and processing formoving the data in the MSIB 11 a written by the WCF processing to the MS11. When the CIB processing is started, it is likely that data movementamong the components (the FS 12, the IS 13, the MS 11, etc.) in the NANDmemory and compaction processing are performed in a chain-reactingmanner. Time required for the overall processing substantially changesaccording to a state.

WCF Processing

First, details of the WCF processing are explained. When a Writecommand, LBA as a writing address, and a writing size is input from theATA-command processing unit 121, the DRAM-layer managing unit 120 asearches through the WC cluster management table 25 shown in FIG. 10(steps S300 and S305). A state of the WC 21 is defined by the state flag25 a (e.g., 3 bits) of the WC cluster management table 25 shown in FIG.10. Most typically, a state of the state flag 25 a transitions in theorder of invalid (usable)→a wait for writing from an ATA→valid(unusable)→a wait for flush to an NAND→invalid (usable). First, aline ata writing destination is determined from logical cluster address LSB(k−i) bits of the LBA and n ways of the determined line are searched.When the logical track address 25 c same as that of the input LBA isstored in the n ways of the determined lines (step S305), the DRAM-layermanaging unit 120 a secures this entry as an entry for writing clusterbecause the entry is to be overwritten (valid (unusable)—a wait forwriting from an ATA).

The DRAM-layer managing unit 120 a notifies the ATA-command processingunit 121 of a DRAM address corresponding to the entry. When writing bythe ATA-command processing unit 121 is finished, the data managing unit120 changes the state flag 25 a of the entry to valid (unusable) andregisters required data in spaces of the sector position bitmap 25 b andthe logical track address 25 c. The data managing unit 120 updates theWC track management table 24. Specifically, when an LBA address same asthe logical track address 24 a already registered in the lists of the WCtrack management table 24 is input, the data managing unit 120 updatesthe number of WC clusters 24 b and the way-line bitmap 24 c of arelevant list and changes the next pointer 24 d such that the listbecomes a latest list. When an LBA address different from the logicaltrack address 24 a registered in the lists of the WC track managementtable 24 is input, the data managing unit 120 creates a new list havingthe entries of the logical track address 24 a, the number of WC clusters24 b, the way-line bitmap 24 c, and the next pointer 24 d and registersthe list as a latest list. The data managing unit 120 performs the tableupdate explained above to complete the write processing (step S320).

On the other hand, when the logical track address 25 c same as that ofthe input LBA is not stored in the n ways of the determined line, thedata managing unit 120 judges whether flush to the NAND memory 10 isnecessary (step S305). First, the data managing unit 120 judges whethera writable way in the determined line is a last nth way. The writableway is a way having the state flag 25 a of invalid (usable) or a wayhaving the state flag 25 a of valid (unusable) and a wait for flush to aNAND. When the state flag 25 a is a wait for flush to a NAND, this meansthat flush is started and an entry is a wait for the finish of theflush. When the writable way is not the last nth way and the writableway is a way having the state flag 25 a of invalid (usable), the datamanaging unit 120 secures this entry as an entry for cluster writing(invalid (usable)→a wait for writing from an ATA). The data managingunit 120 notifies the ATA-command processing unit 121 of a DRAM addresscorresponding to the entry and causes the ATA-command processing unit121 to execute writing. In the same manner as explained above, the datamanaging unit 120 updates the WC cluster management table 25 and the WCtrack management table 24 (step S320).

When the writable way is not the last nth way and when the writable wayis the way having the state flag 25 a of valid (unusable) and a wait forflush to a NAND, the data managing unit 120 secures this entry as anentry for writing cluster (valid (unusable) and a wait for flush to aNAND→valid (unusable) and a wait for flush from a NAND and a wait forwriting from an ATA). When the flush is finished, the data managing unit120 changes the state flag 25 a to a wait for writing from an ATA,notifies the ATA-command processing unit 121 of a DRAM addresscorresponding to the entry, and causes the ATA-command processing unit121 to execute writing. In the same manner as explained above, the datamanaging unit 120 updates the WC cluster management table 25 and the WCtrack management table 24 (step S320).

The processing explained above is performed when flush processing doesnot have to be triggered when a writing request from the ATA-commandprocessing unit 121 is input. On the other hand, processing explainedbelow is performed when flush processing is triggered after a writingrequest is input. At step S305, when the writable way in the determinedline is the last nth way, the data managing unit 120 selects track to beflushed, i.e., an entry in the WC 21 based on the condition explained in(i) of the method of determining data to be flushed from the WC 21 tothe NAND memory 10, i.e.,

(i) when a writable way determined by a tag is a last (in thisembodiment, nth) free way, i.e., when the last free way is to be used,track updated earliest based on an LRU among track registered in theline is decided to be flushed.

When that track to be flushed is determined according to the policyexplained above, as explained above, if all cluster in the WC 21included in an identical logical track address are to be flushed and anamount of cluster to be flushed exceeds 50% of a track size, i.e., ifthe number of valid cluster in the WC is equal to or larger than2^((k−i−l)) in the track decided to be flushed, the DRAM-layer managingunit 120 a performs flush to the MSIB 11 a (step S310). If the amount ofcluster does not exceeds 50% of the track size, i.e., the number ofvalid cluster in the WC is smaller than 2^((k−i−l)) in the track decidedto be flushed, the DRAM-layer managing unit 120 a performs flush to theFSIB 12 a (step S315). Details of the flush from the WC 21 to the MSIB11 a and the flush from the WC 21 to the FSIB 12 a are explained later.The state flag 25 a of the selected flush entry is transitioned fromValid (unusable) to a wait for flush to the NAND memory 10.

This judgment on a flush destination is executed by using the WC trackmanagement table 24. An entry of the number of WC clusters 24 indicatingthe number of valid clusters is registered in the WC track managementtable 24 for each logical track address. The data managing unit 120determines which of the FSIB 12 a and the MSIB 11 a should be set as adestination of flush from the WC 21 referring to the entry of the numberof WC clusters 24 b. All clusters belonging to the logical track addressare registered in a bitmap format in the way-line bitmap 24 c.Therefore, in performing flush, the data managing unit 120 can easilylearn, referring to the way-line bitmap 24 c, a storage position in theWC 21 of each of the cluster that should be flushed.

During the write processing or after the write processing, the datamanaging unit 120 also execute the flush processing to the NAND memory10 in the same manner when the following condition is satisfied:

(ii) the number of tracks registered in the WC 21 exceeds apredetermined number.

WC→MSIB (Copy)

When flush from the WC 21 to the MSIB 11 a is performed according to thejudgment based on the number of valid clusters (the number of validclusters is equal to or larger than 2^((k−i−l))), the data managing unit120 executes a procedure explained below as explained above (step S310).

1. Referring to the WC cluster management table 25 and referring to thesector position bitmaps 25 b in tags corresponding to cluster to beflushed, when all the sector position bitmaps 25 b are not “1”, the datamanaging unit 120 performs intra-track sector padding explained laterfor merging with sector not present in the WC 21 by reading out themissing sector included in the identical logical track address from theNAND memory 10.

2. When the number of tracks decided to be flushed is less than 2^(i),the data managing unit 120 adds tracks decided to be flushed having2^((k−i−l)) or more valid clusters until the number of tracks decided tobe flushed reaches 2^(i) from the oldest one in the WC 21.

3. When there are 2^(i) or more tracks to be copied, the data managingunit 120 performs writing in the MSIB 11 a in logical block units witheach 2^(i) tracks as a set.

4. The data managing unit 120 writes the tracks that cannot form a setof 2^(i) tracks in the MSIB 11 a in track units.

5. The data managing unit 120 invalidates clusters and tracks belongingto the copied tracks among those already present on the FS, the IS, andthe MS after the Copy is finished.

Update processing for the respective management tables involved in theCopy processing from the WC 21 to the MSIB 11 a is explained. The datamanaging unit 120 sets the state flag 25 a in entries corresponding toall clusters in the WC 21 belonging to a flushed track in the WC clustermanagement table 25 Invalid. Thereafter, writing in these entries ispossible. Concerning a list corresponding to the flushed track in the WCtrack management table 24, the data managing unit 120 changes ordeletes, for example, the next pointer 24 d of an immediately precedinglist and invalidates the list.

On the other hand, when track flush from the WC 21 to the MSIB 11 a isperformed, the data managing unit 120 updates the track management table30 and the MS logical block management table 35 according to the trackflush. First, the data managing unit 120 searches for the logical trackaddress 30 a as an index of the track management table 30 to judgewhether the logical track address 30 a corresponding to the flushedtrack is already registered. When the logical track address 30 a isalready registered, the data managing unit 120 updates fields of thecluster bitmap 30 b (because the track is flushed to the MS 11 side, allrelevant bits are set to “0”) of the index and the logical block ID 30c+the intra-logical block track position 30 d. When the logical trackaddress 30 a corresponding to the flushed track is not registered, thedata managing unit 120 registers the cluster bitmap 30 b and the logicalblock ID 30 c+the intra-logical block track position 30 d in an entry ofthe relevant logical track address 30 a. The data managing unit 120updates, according to the change of the track management table 30,entries of the logical block ID 35 a, the track management pointer 35 b,the number of valid tracks 35 c, the writable top track 35 d, and thelike in the MS logical block management table 35 when necessary.

When track writing is performed from other areas (the FS 12 and the IS13) or the like to the MS 11 or when intra-MS track writing bycompaction processing in the MS 11 is performed, valid clusters in theWC 21 included in the logical track address as a writing object may besimultaneously written in the MS 11. Such passive merge may be presentas writing from the WC 21 to the MS 11. When such passive merge isperformed, the clusters are deleted from the WC 21 (invalidated).

WCΔFSIB (Copy)

When flush from the WC 21 to the FSIB 12 a is performed according to thejudgment based on the number of valid clusters (the number of validclusters is equal to or larger than 2^((k−i−l))), the data managing unit120 executes a procedure explained below.

1. Referring to the sector position bitmaps 25 b in tags correspondingto clusters to be flushed, when all the sector position bitmaps 25 b arenot “1”, the data managing unit 120 performs intra-cluster sectorpadding for merging with sector not present in the WC 21 by reading outthe missing sector included in the identical logical cluster addressfrom the NAND memory 10.

2. The data managing unit 120 extracts clusters from a track having onlyless than 2^((k−i−l)) valid clusters tracing tracks in the WC in orderfrom oldest one and, when the number of valid clusters reaches 2^(k),writes all the clusters in the FSIB 12 a in logical block units.

3. When 2^(k) valid clusters are not found, the data managing unit 120writes all track with the number of valid clusters less than 2^((k−i−l))in the FSIB 12 a by the number equivalent to the number of logicalpages.

4. The data managing unit 120 invalidates clusters with same logicalcluster address as the clusters copied among those already present onthe FS 12 and the IS 13 after the Copy is finished.

Update processing for the respective management tables involved in suchCopy processing from the WC 21 to the FSIB 12 a is explained. The datamanaging unit 120 sets the state flag 25 a in entries corresponding toall clusters in the WC 21 belonging to a flushed track in the WC clustermanagement table 25 Invalid. Thereafter, writing in these entries ispossible. Concerning a list corresponding to the flushed track in the WCtrack management table 24, the data managing unit 120 changes ordeletes, for example, the next pointer 24 d of an immediately precedinglist and invalidates the list.

On the other hand, when cluster flush from the WC 21 to the FSIB 12 a isperformed, the data managing unit 120 updates the cluster table pointer30 e, the number of FS clusters 31 f, and the like of the trackmanagement table 30 according to the cluster flush. The data managingunit 120 also updates the logical block ID 40 b, the intra-logical blockcluster position 40 c, and the like of the FS/IS management table 40.Concerning clusters not present in the FS 12 originally, the datamanaging unit 120 adds a list to the linked list of the FS/IS managementtable 40. According to the update, the data managing unit 120 updatesrelevant sections of the MS logical block management table 35, the FS/ISlogical block management table 42, and the intra-FS/IS clustermanagement table 44.

CIB Processing

When the WCF processing explained above is finished, thelogical-NAND-layer managing unit 120 b executes CIB processing includingprocessing for moving the data in the FSIB 12 a written by the WCFprocessing to the FS 12 and processing for moving the data in the MSIB11 a written by the WCF processing to the MS 11. When the CIB processingis started, as explained above, it is likely that data movement amongthe blocks and compaction processing are performed in a chain reactingmanner. Time required for the overall processing substantially changesaccording to a state. In the CIB processing, basically, first, the CIBprocessing in the MS 11 is performed (step S330), subsequently, the CIBprocessing in the FS 12 is performed (step S340), the CIB processing inthe MS 11 is performed again (step S350), the CIB processing in the IS13 is performed (step S360), and, finally, the CIB processing in the MS11 is performed again (step S370). In flush processing from the FS 12 tothe MSIB 11 a, flush processing from the FS 12 to the IS 13, or flushprocessing from the IS 13 to the MSIB 11 a, when a loop occurs in aprocedure, the processing may not be performed in order. The CIBprocessing in the MS 11, the CIB processing in the FS 12, and the CIBprocessing in the IS 13 are separately explained.

CIB Processing in the MS 11

First, the CIB processing in the MS 11 is explained (step S330). Whenmovement of track from the WC 21, the FS 12, and the IS 13 to the MS 11is performed, the track is written in the MSIB 11 a. After thecompletion of writing in the MSIB 11 a, as explained above, the trackmanagement table 30 is updated and the logical block ID 30 c, theintra-block track position 30 d, and the like in which tracks arearranged are changed (Move). When new track is written in the MSIB 11 a,track present in the MS 11 or the TFS 11 b from the beginning isinvalidated. This invalidation processing is realized by invalidating atrack from an entry of a logical block in which old track information isstored in the MS logical block management table 35. Specifically, apointer of a relevant track in a field of the track management pointer35 b in the entry of the MS logical block management table 35 is deletedand the number of valid tracks is decremented by one. When all tracks inone logical block are invalidated by this track invalidation, the validflag 35 e is invalidated. Logical blocks of the MS 11 including invalidtracks are generated by such invalidation or the like. When this isrepeated, efficiency of use of logical blocks may fall to causeinsufficiency in usable logical blocks.

When such a situation occurs and the number of logical blocks allocatedto the MS 11 exceeds the upper limit of the number of logical blocksallowed for the MS 11, the data managing unit 120 performs compactionprocessing to create a free block FB. The free block FB is returned tothe physical-NAND-layer managing unit 120 c. The logical-NAND-layermanaging unit 120 b reduces the number of logical blocks allocated tothe MS 11 and, then, acquires a writable free block FB from thephysical-NAND-layer managing unit 120 c anew. The compaction processingis processing for collecting valid clusters of a logical block as acompaction object in a new logical block or copying valid tracks in thelogical block as the compaction object to other logical blocks to createa free block FB returned to the physical-NAND-layer managing unit 120 cand improve efficiency of use of logical blocks. In performingcompaction, when valid clusters on the WC 21, the FS 12, and the IS 13are present, the data managing unit 120 executes passive merge formerging all the valid clusters included in a logical track address as acompaction object. Logical blocks registered in the TFS 11 b are notincluded in the compaction object.

An example of Move from the MSIB 11 a to the MS 11 or to the TFS 11 band compaction processing with presence of a full logical block in theMSIB 11 a set as a condition is specifically explained. The “full”logical block means the logical block in which all logical pages hasbeen written and additional recording is impossible.

1. Referring to the valid flag 35 e of the MS logical block managementtable 35, when an invalidated logical block is present in the MS 11, thedata managing unit 120 sets the logical block as a free block FB.

2. The data managing unit 120 moves a full logical block in the MSIB 11a to the MS 11. Specifically, the data managing unit 120 updates the MSstructure management table (not shown) explained above and transfers thelogical block from management under the MSIB 11 a to management underthe MS 11.

3. The data managing unit 120 judges whether the number of logicalblocks allocated to the MS 11 exceeds the upper limit of the number oflogical blocks allowed for the MS 11. When the number of logical blocksexceeds the upper limit, the data managing unit 120 executes MScompaction explained below.

4. Referring to a field and the like of the number of valid tracks 35 cof the MS logical block management table 35, the data managing unit 120sorts logical blocks having invalidated tracks among logical blocks notincluded in the TFS 11 b with the number of valid tracks.

5. The data managing unit 120 collects tracks from logical blocks withsmall numbers of valid tracks and carries out compaction. In carryingout compaction, first, the tracks are copied for each of the logicalblocks (2^(i) tracks are copied at a time) to carry out compaction. Whena track as a compaction object has valid clusters in the WC 21, the FS12, and the IS 13, the data managing unit 120 also merges the validclusters.

6. The data managing unit 120 sets the logical block at a compactionsource as a free block FB.

7. When the compaction is performed and one logical block includes thevalid 2^(i) tracks, the data managing unit 120 moves the logical blockto the top of the TFS 11 b.

8. When the free block FB can be created by copying the valid tracks inthe logical block to another logical block, the data managing unit 120additionally records the valid tracks in the number smaller than 2^(i)in the MSIB 11 a in track units.

9. The data managing unit 120 sets the logical block at the compactionsource as the free block FB.

10. When the number of logical blocks allocated to the MS 11 falls belowthe upper limit of the number of logical blocks allowed for the MS 11,the data managing unit 120 finishes the MS compaction processing.

CIB Processing in the FS 12

The CIB processing in the FS 12 is explained (step S340). When fulllogical blocks in which all logical pages are written are created in theFSIB 12 a by cluster writing processing from the WC 21 to the FSIB 12 a,the logical blocks in the FSIB 12 a are moved from the FSIB 12 a to theFS 12. According to the movement, an old logical block is flushed fromthe FS 12 of the FIFO structure configured by a plurality of logicalblocks.

Flush from the FSIB 12 a to the FS 12 and flush from the FS 12 to the MS11 and/or the IS 13 are specifically realized as explained below.

1. Referring to the valid flag 35 e and the like of the FS/IS logicalblock management table 42, when an invalidated logical block is presentin the FS 12, the data managing unit 120 sets the logical block as afree block FB.

2. The data managing unit 120 flushes a full logical block in the FSIB12 a to the FS 12. Specifically, the data managing unit 120 updates theFS/IS structure management table (not shown) and transfers the logicalblock from management under the FSIB 12 a to management under the FS 12.

3. The data managing unit 120 judges whether the number of logicalblocks allocated to the FS 12 exceeds the upper limit of the number oflogical blocks allowed for the FS 12. When the number of logical blocksexceeds the upper limit, the data managing unit 120 executes flushexplained below.

4. The data managing unit 120 determines cluster that should be directlycopied to the MS 11 without being moving to the IS 13 among clusters inan oldest logical block as an flush object (actually, because amanagement unit of the MS 11 is a track, the cluster is determined intrack units).

-   -   (A) The data managing unit 120 scans valid clusters in the        oldest logical block as the flush object in order from the top        of a logical page.    -   (B) The data managing unit 120 finds, referring to a field of        the number of FS clusters 30 f of the track management table 30,        how many valid clusters a track to which the cluster belongs has        in the FS 12.    -   (C) When the number of valid clusters in the track is equal to        or larger than a predetermined threshold (e.g., 50% of 2^(k−l)),        the data managing unit 120 sets the track as a candidate of        flush to the MS 11.

5. The data managing unit 120 writes the track that should be flushed tothe MS 11 in the MSIB 11 a.

6. When valid clusters to be flushed in the track units are left in theoldest logical block, the data managing unit 120 further executes flushto the MSIB 11 a.

7. When valid clusters are present in the logical block as the flushobject even after the processing of 2 to 4 above, the data managing unit120 moves the oldest logical block to the IS 13.

When flush from the FS 12 to the MSIB 11 a is performed, immediatelyafter the flush, the data managing unit 120 executes the CIB processingin the MS 11 (step s350).

CIB Processing in the IS 13

The CIB processing in the IS 13 is explained (step S360). The logicalblock is added to the IS 13 according to the movement from the FS 12 tothe IS 13. However, according to the addition of the logical block, thenumber of logical blocks exceeds an upper limit of the number of logicalblocks that can be managed in the IS 13 formed of a plurality of logicalblocks. When the number of logical blocks exceeds the upper limit, inthe IS 13, the data managing unit 120 performs flush of one to aplurality of logical blocks to the MS 11 and executes IS compaction.Specifically, the data managing unit 120 executes a procedure explainedbelow.

1. The data managing unit 120 sorts tracks included in the IS 13 withthe number of valid clusters in the track×a valid cluster coefficient,collects 2^(i+1) tracks (for two logical blocks) with a large value of aproduct, and flushes the tracks to the MSIB 11 a.

2. When a total number of valid clusters of 2^(i+1) logical blocks witha smallest number of valid clusters is, for example, equal to or largerthan 2^(k) (for one logical block), which is a predetermined set value,the data managing unit 120 repeats the step explained above.

3. After performing the flush, the data managing unit 120 collects 2^(k)clusters in order from a logical block with a smallest number of validclusters and performs compaction in the IS 13.

4. The data managing unit 120 releases a logical block not including avalid cluster among the logical blocks at compaction sources as a freeblock FB.

When flush from the IS 13 to the MSIB 11 a is performed, immediatelyafter the flush, the data managing unit 120 executes the CIB processingin the MS 11 (step S370).

FIG. 20 is a diagram of combinations of inputs and outputs in a flow ofdata among components and indicates what causes the flow of the data asa trigger. Basically, data is written in the FS 12 according to clusterflush from the WC 21. However, when intra-cluster sector padding(cluster padding) is necessary incidentally to flush from the WC 21 tothe FS 12, data from the FS 12, the IS 13, and the MS 11 is copied. Inthe WC 21, it is possible to perform management in sector (512 B) unitsby identifying presence or absence of 2^((l−k)) sectors in a relevantlogical cluster address using the sector position bitmap 25 b in the tagof the WC cluster management table 25. On the other hand, a managementunit of the FS 12 and the IS 13, which are functional components in theNAND memory 10, is a cluster and a management unit of the MS 11 is atrack. In this way, a management unit in the NAND memory 10 is largerthan the sector. Therefore, in writing data in the NAND memory 10 fromthe WC 21, when data with a logical cluster address identical with thatof the data to be written is present in the NAND memory 10, it isnecessary to write the data in the NAND memory 10 after merging a sectorin a cluster to be written in the NAND memory 10 from the WC 21 and asector in the identical logical cluster address present in the NANDmemory 10.

This processing is the intra-cluster sector padding processing (thecluster padding) and the intra-track sector padding (the track padding)shown in FIG. 20. Unless these kinds of processing are performed,correct data cannot be read out. Therefore, when data is flushed fromthe WC 21 to the FSIB 12 a or the MSIB 11 a, the WC cluster managementtable 25 is referred to and the sector position bitmaps 25 b in tagscorresponding to clusters to be flushed is referred to. When all thesector position bitmaps 25 b are not “1”, the intra-cluster sectorpadding or the intra-track sector padding for merging with a sector inan identical cluster or an identical track included in the NAND memory10 is performed. A work area of the DRAM 20 is used for this processing.A plurality of sectors included in a logical cluster address or alogical track address is merged on the work area of the DRAM 20 and dataimage (cluster image or track image) to be flushed is created. Thecreated data image is written in the MSIB 11 a or written in the FSIB 12a from the work area of the DRAM 20.

In the IS 13, basically, data is written according to block flush fromthe FS 12 (Move) or written according to compaction in the IS. In the MS11, data can be written from all sections. When the data is written, inthe MS 11, padding due to data of the MS itself can be caused becausedata can only be written in track units. When the data is written intrack units, fragmented data in other logical blocks are also writtenaccording to passive merge. Moreover, in the MS 11, data is also writtenaccording to MS compaction. In the passive merge, when track flush orlogical block flush (flush for 2^(i) tracks) from one of threecomponents of the WC 21, the FS 12, or the IS 13 to the MS 11 isperformed, valid clusters in two components included in a track (or alogical block) as an flush object in one component and valid clusters inthe MS 11 are collected in the work area of the DRAM 20 and written inthe MSIB 11 a from the work area of the DRAM 20 as data for one track.An essential part of the memory system according to this embodiment isexplained with reference to FIGS. 21 and 22. Processing shown in FIGS.21 and 22 is controlled to be executed by the data managing unit 120(see FIG. 4) that receives a request from the host apparatus.

As explained in the section of the background art, it is a main purposeof the method in the past to improve durability against the readdisturb. Therefore, measures for the purpose are complicated and asubstantial increase in cost is inevitably caused. On the other hand, ina secondary storage device employing an NAND flash memory in which dataerasing, writing, and readout are frequently performed, it is oftenrealistically difficult to totally eliminate the read disturb itself.

There is a concept of durable life in the storage device of this type.If a normal operation during a durable life period can be surelyguaranteed, the storage device can play a role thereof. Therefore, inthe NAND flash memory used as a large-capacity secondary storage device,if the occurrence of the read disturb can be delayed, it is possible tosuppress the progress of deterioration in the durable life of thestorage device due to the read disturb.

(Processing for Reducing NAND Memory Accesses During Read Processing)

FIG. 21 is a flowchart of a processing flow of processing for reducingNAND memory accesses during read processing. In FIG. 21, the datamanaging unit 120 receives a Read request from the host apparatus (stepST101). The data managing unit 120 refers to, based on an LBA designatedby the host apparatus, the WC cluster management table 25 (see FIGS. 8and 10), the RC cluster management table 23 (see FIGS. 8 and 9), the WCtrack management table 24 (see FIGS. 8 and 11), the track managementtable 30 (see FIGS. 8 and 12), the FS/IS management table 40 (see FIGS.8 and 13), and the like and determines whether an entry is present inthe WC 21, the RC 22, the MS 11, the FS 12, or the IS 13 (step ST102).

When the entry of the LBA designated by the host apparatus 1(hereinafter referred to as “designated LBA”) (“Yes” at step ST102), thedata managing unit 120 reads out a relevant data from a storage locationcorresponding thereto in the NAND memory 10 (step ST103), notifies thehost apparatus 1 of the data (step ST104), and finishes the processing.

On the other hand, when the entry of the designated LBA is not present(“No” at step ST102), the data managing unit 120 notifies the hostapparatus 1 of data read out from a dummy data area explained later(step ST105) and finishes the overall processing. The entry of thedesignated LBA is not present when a valid entry is not present on theFS/IS management table 40 and a logical block ID is an invalid value (isnot mapped to the MS 11 yet) on the track management table.

(Dummy Data and a Dummy Data Area)

Dummy data and a dummy data area are explained. The dummy data is datahaving a predetermined value that the data managing unit 120 shouldreturn to the host apparatus side (the ATA-command processing unit 121)when a state of the NAND memory 10 meets a predetermined condition. Dataof a cluster size is prepared to pad a cluster. As the dummy data, forexample, “all 0” [0x0000 . . . ] and “all 1” [0xFFFF . . . ] can beused.

The dummy data area is a storage area in which the dummy data is stored.In this embodiment, as explained above, the data managing unit 120notifies the ATA command processing unit 121 of the dummy data or anaddress of the dummy data area instead of a message for notifying thatthe LBA designated by the host apparatus (the designated LBA) is an LBAarea not written on the NAND memory 10 (an unwritten LBA area).

(Processing for Reducing NAND Memory Accesses During Write Processing)

FIG. 22 is a flowchart of a processing flow of processing for reducingNAND memory accesses during Write processing. In FIG. 22, when a writerequest requiring data flushing from the WC 21 (a flushing request) isgenerated (step ST201) and when all data of a logical address areaincluding a valid cluster decided to be flushed are not present in theWC 21, intra-cluster sector padding and intra-track sector padding occur(step ST202).

The data managing unit 120 refers to the WC cluster management table 25.However, when all sector position bitmaps 25 b in a tag corresponding toa cluster to be flushed are not “1”, the sector position bitmaps 25 bneed to be merged with sectors in the same cluster included in the NANDmemory 10. Therefore, the data managing unit 120 executes processing fordetermining whether an entry of a corresponding address area that shouldbe used for the padding is present in the NAND memory 10 (step ST203).

When the entry of the corresponding address area used for theintra-cluster sector padding and the intra-track sector padding involvedin the flushing from the WC 21 is present in the NAND memory 10 (“Yes”at step ST203), the data managing unit 120 selects a sector in the samecluster stored in the NAND memory 10 as a padding source and merged witha sector flushed from the WC 21 (step ST204). The data managing unit 120writes cluster data padded with the valid data in the NAND memory in apredetermined location in the NAND memory 10 (step ST205) and finishesthis processing flow.

On the other hand, when the entry of the corresponding address area thatshould be used for the intra-cluster sector padding and the intra-tracksector padding involved in the flushing from the WC 21 is not present inthe NAND memory 10 (“No” at step ST203), the data managing unit 120selects dummy data stored in the DRAM 20 as a padding source (stepST206). The data managing unit 120 writes cluster data padded with thedummy data in a predetermined location in the NAND memory 10 (step S207[translator's comment: “S207” should be corrected to “ST207”]) andfinishes this processing flow.

(Supplementary Explanation and Effects by this Embodiment)

The processing explained above is briefly explained supplementarily. Inthe memory system according to this embodiment, as explained above, theprocessing for returning the data of the dummy data area on the DRAM 20is performed in response to the Read request and the Write requestincluding the unwritten LBA. This processing makes it possible toautomatically return determined fixed data to the host side withoutmanaging which sector area is unwritten after writing. Consequently, itis possible to reduce a memory capacity for managing the unwritten LBAarea.

When a readout request for the unwritten LBA area is generated duringRead processing, the data managing unit 120 returns data read out fromthe dummy data area on the DRAM 20 without accessing the NAND memory 10.Therefore, it is possible to reduce the number of times of unnecessaryreadout processing for the NAND memory 10. As a result, it is possibleto delay occurrence of the read disturb and suppress the progress ofdeterioration in the durable life of the storage device due to the readdisturb.

When the entry of the corresponding address area that should be used forthe intra-cluster sector padding and the intra-track sector paddinginvolved in the flushing from the WC 21 is not present in the NANDmemory 10 during Write processing, the data managing unit 120 selectsthe dummy data area on the DRAM 20 as a padding source. Therefore, it ispossible to reduce the number of times of unnecessary readout processingfor the NAND memory 10. As a result, it is possible to delay occurrenceof the read disturb and suppress the progress of deterioration in thedurable life of the storage device due to the read disturb.

In this embodiment, values such as “all 0” [0x0000 . . . ] and “all 1”[0xFFFF . . . ] are used as the dummy data. However, the dummy data isnot limited to these values. Arbitrary values can be used as the dummydata.

As explained above, in this embodiment, it is possible to suppress theprogress of deterioration in the durable life of the storage device dueto the read disturb.

The present invention is not limited to the embodiments described above.Accordingly, various modifications can be made without departing fromthe scope of the present invention.

Furthermore, the embodiments described above include variousconstituents with inventive step. That is, various modifications of thepresent invention can be made by distributing or integrating anyarbitrary disclosed constituents.

For example, various modifications of the present invention can be madeby omitting any arbitrary constituents from among all constituentsdisclosed in the embodiments as long as problem to be solved by theinvention can be resolved and advantages to be attained by the inventioncan be attained.

Furthermore, it is explained in the above embodiments that a clustersize multiplied by a positive integer equal to or larger than two equalsto a logical page size. However, the present invention is not to be thuslimited.

For example, the cluster size can be the same as the logical page size,or can be the size obtained by multiplying the logical page size by apositive integer equal to or larger than two by combining a plurality oflogical pages.

Moreover, the cluster size can be the same as a unit of management for afile system of OS (Operating System) that runs on the host apparatus 1such as a personal computer.

Furthermore, it is explained in the above embodiments that a track sizemultiplied by a positive integer equal to or larger than two equals to alogical block size. However, the present invention is not to be thuslimited.

For example, the track size can be the same as the logical block size,or can be the size obtained by multiplying the logical block size by apositive integer equal to or larger than two by combining a plurality oflogical blocks.

If the track size is equal to or larger than the logical block size, MScompaction processing is not necessary. Therefore, the TFS 11 b can beomitted.

Second Embodiment

FIG. 23 shows a perspective view of an example of a personal computer. Apersonal computer 1200 includes a main body 1201 and a display unit1202. The display unit 1202 includes a display housing 1203 and adisplay device 1204 accommodated in the display housing 1203.

The main body 1201 includes a chassis 1205, a keyboard 1206, and a touchpad 1207 as a pointing device. The chassis 1205 includes a main circuitboard, an ODD unit (Optical Disk Device), a card slot, and the SSD 1100described in the first embodiment.

The card slot is provided so as to be adjacent to the peripheral wall ofthe chassis 1205. The peripheral wall has an opening 1208 facing thecard slot. A user can insert and remove an additional device into andfrom the card slot from outside the chassis 1205 through the opening1208.

The SSD 1100 may be used instead of the prior art HDD in the state ofbeing mounted in the personal computer 1200 or may be used as anadditional device in the state of being inserted into the card slot ofthe personal computer 1200.

FIG. 24 shows a diagram of an example of system architecture in apersonal computer. The personal computer 1200 is comprised of CPU 1301,a north bridge 1302, a main memory 1303, a video controller 1304, anaudio controller 1305, a south bridge 1309, a BIOS-ROM 1310, the SSD1100 described in the first embodiment, an ODD unit 1311, an embeddedcontroller/keyboard controller (EC/KBC) IC 1312, and a networkcontroller 1313.

The CPU 1301 is a processor for controlling an operation of the personalcomputer 1200, and executes an operating system (OS) loaded from the SSD1100 to the main memory 1303. The CPU 1301 executes these processes,when the ODD unit 1311 executes one of reading process and writingprocess to an optical disk. The CPU 1301 executes a system BIOS (BasicInput Output System) stored in the BIOS-ROM 1310. The system BIOS is aprogram for controlling a hard ware of the personal computer 1200.

The north bridge 1302 is a bridge device which connects the local bus ofthe CPU 1301 to the south bridge 1309. The north bridge 1302 has amemory controller for controlling an access to the main memory 1303. Thenorth bridge 1302 has a function which executes a communication betweenthe video controller 1304 and the audio controller 1305 through the AGP(Accelerated Graphics Port) bus.

The main memory 1303 stores program or data temporary, and functions asa work area of the CPU 1301. The main memory 1303 is comprised of, forexample, DRAM. The video controller 1304 is a video reproduce controllerfor controlling a display unit which is used for a display monitor (LCD)1316 of the portable computer 1200. The Audio controller 1305 is anaudio reproduce controller for controlling a speaker of the portablecomputer 1200.

The south bridge 1309 controls devices connected to the LPC (Low PinCount) bus, and controls devices connected to the PCI (PeripheralComponent Interconnect) bus. The south bridge 1309 controls the SSD 1100which is a memory device stored soft ware and data, through the ATAinterface.

The personal computer 1200 executes an access to the SSD 1100 in thesector unit. For example, the write command, the read command, and thecache flash command are input through the ATA interface. The southbridge 1309 has a function which controls the BIOS-ROM 1310 and the ODDunit 1311.

The EC/KBC 1312 is one chip microcomputer which is integrated on theembedded controller for controlling power supply, and the key boardcontroller for controlling the key board (KB) 1206 and the touch pad1207. The EC/KBC 1312 has a function which sets on/off of the powersupply of the personal computer 1200 based on the operation of the powerbutton by user. The network controller 1313 is, for example, acommunication device which executes the communication to the network,for example, the internet.

Although the memory system in the above embodiments is comprised as anSSD, it can be comprised as, for example, a memory card typified by anSD™ card. Moreover, the memory system can be applied not only to apersonal computer but also to various electronic devices such as acellular phone, a PDA (Personal Digital Assistant), a digital stillcamera, a digital video camera, and a television set.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A memory system comprising: a first storing area as a cache memoryincluding a volatile semiconductor memory; second and third storing areaincluding nonvolatile semiconductor memories in which readout andwriting are performed in a page unit and erasing is performed in a blockunit twice or a larger natural number times as large as the page unit; amanagement table group in which management information including storagelocations of data stored in the first, second, and third storing areasis stored; and a controller that allocates a storage area of thenonvolatile semiconductor memory to the second and third storing areasin a logical block unit associated with one or more of the blocks,wherein when a readout request is received from outside, the controllerdetermines, based on the management table group, whether an unwrittenlogical address area is present in the second or third storage area towhich a logical address area requested to be read out is mapped andnotifies, when the unwritten logical address area is present,predetermined fixed data to the outside in association with theunwritten logical address area.
 2. The memory system according to claim1, wherein the controller manages the first and second storing areas ina first management unit and manages the third storing area in a secondmanagement unit.
 3. The memory system according to claim 1, wherein thefixed data is stored in the first storing area.
 4. The memory systemaccording to claim 3, wherein data having a size in the first managementunit is provided as the fixed data.
 5. The memory system according toclaim 4, wherein data having bit values of “0” or “1” is provided as thefixed data.
 6. The memory system according to claim 2, wherein the firstmanagement unit is a size obtained by dividing the page unit by anatural number.
 7. The memory system according to claim 2, wherein thesecond management unit is a size twice or a larger natural number timesas large as the page unit and obtained by dividing the block unit by anatural number equal to or larger than two.
 8. The memory systemaccording to claim 1, wherein the volatile semiconductor memory is aDRAM, and the nonvolatile semiconductor memory is a NAND flash memory.9. A memory system comprising: a first storing area as a cache memoryincluding a volatile semiconductor memory; second and third storingareas including nonvolatile semiconductor memories in which readout andwriting are performed in a page unit and erasing is performed in a blockunit twice or a larger natural number times as large as the page unit; amanagement table group in which management information including storagelocations of data stored in the first, second, and third storing areasis stored; and a controller that allocates a storage area of thenonvolatile semiconductor memory to the second and third storing areasin a logical block unit associated with one or more of the blocks,wherein when a write request involving data flushing from the firststoring area to the second storing area is received from outside and alldata of a logical address area including the page unit flushed from thefirst storing area to the second storing area are not present in thefirst storing area, the controller determines, based on the managementtable, whether an unwritten logical address area is present in thesecond and third storing areas to which a logical address area used forpadding data not present in the first storing area is mapped andselects, when the unwritten logical address area is present,predetermined fixed data as padding source data.
 10. The memory systemaccording to claim 9, wherein the controller manages the first andsecond storing areas in a first management unit and manages the thirdstoring area in a second management unit.
 11. The memory systemaccording to claim 10, wherein the controller executes first processingfor writing a plurality of data in a sector unit in the first storingarea as data in the first management unit, second processing forflushing the data written in the first storing area to the secondstoring area as the data in the first management unit, and thirdprocessing for flushing the data written in the first storing area tothe third storing area as data in the second management unit.
 12. Thememory system according to claim 11, wherein the controller executes thesecond or third processing when a number of data in the secondmanagement unit to which the data in the first storing area belongsexceeds a specified value.
 13. The memory system according to claim 11,wherein, in executing the second processing, when data having an addresssame as that of the data in the first management unit flushed from thefirst storing area is present in the second storing area, the controllerwrites, after merging sector data present in the first storing area andsector data present in the second storing area in the same address, thesector data in the second storing area.
 14. The memory system accordingto claim 11, wherein, in executing the third processing, when datahaving an address same as that of the data in the first management unitflushed from the first storing area is present in the third storingarea, the controller writes, after merging sector data present in thefirst storing area and sector data present in the third storing area inthe same address, the sector data in the third storing area.
 15. Thememory system according to claim 9, wherein the fixed data is stored inthe first storing area.
 16. The memory system according to claim 15,wherein data having a size in the first management unit is provided asthe fixed data.
 17. The memory system according to claim 16, whereindata having bit values of “0” or “1” is provided as the fixed data. 18.The memory system according to claim 10, wherein the first managementunit is a size obtained by dividing the page unit by a natural number.19. The memory system according to claim 10, wherein the secondmanagement unit is a size twice or a larger natural number times aslarge as the page unit and obtained by dividing the block unit by anatural number equal to or larger than two.
 20. The memory systemaccording to claim 9, wherein the volatile semiconductor memory is aDRAM, and the nonvolatile semiconductor memory is a NAND flash memory.